One factor in the sharpness of a picture is movement on the part of the photographer (“camera shake”). This is especially true at longer focal lengths. A rule of thumb from 35 mm film photography is that hand-held exposures should be less than one over the focal length in seconds.
One way of addressing this limitation is to use a faster lens, allowing shorter exposure time for the same scene brightness. Digital cameras, however, already use the fastest lens practical, consistent with cost, size and image quality goals. Values of F/2 to F/2.8 are typical. Faster lenses than these are much more bulky and expensive.
Active image stabilization of the lens can also be used to minimize camera shake. In an image-stabilized lens, motion is sensed (e.g., by a solid-state gyroscope) and an optical element within the lens is moved in such a way that the image path is deflected slightly in the direction opposite the camera motion. In one approach, the element is driven by two “voice coil” type actuators, responding to signals from gyros or accelerometers sensing horizontal and vertical motion. One approach has been to use a liquid-filled prism. Actuators squeeze the prism by the edge, refracting the beam in the direction of the thicker side of the prism. Another approach shifts the lenses within the optical path to adjust for movement.
While these approaches work, they take up space within the lens and add bulk and expense. In particular, the liquid prism approach adds a separate, additional element that must be placed into the optical path.
It is also possible to move the sensor itself in response to camera motion. This is difficult, however, because the interconnect to the sensor hinders a compliant suspension in both horizontal and vertical directions. Also, the suspension should be very accurate and stiff in all of the other degrees of freedom, particularly the tilt axes.